Simple take down, line up the notches and push the pivot pin out on the left side. I recently stripped my 639 after about 10 years. Mainly to replace the recoil spring. After thousands of reloads it took a few swaps of #9 in the nooks and corners to make it good as new. A dab of Tetra on the rails and it should be good to go for another 10 years.

Smith & Wesson Semi-Auto Pistols Thread, Instructions on field stripping a 639 in Smith & Wesson Semi-Automatic Pistols; Hello!
I inherited a Model 639 from my grandfather many years ago. I want to take it to target practice, ...